A prior art electrical cable is composed of an inner silver-coated copper wire conductor, a central dielectric insulator layer, an outer silver-coated copper shield, and an external dielectric jacket. FIG. 1 shows a cross-section of a prior art cable.
When this prior art cable is exposed to transient X-rays and gamma-rays from a nuclear explosion, that is high energy photons, large numbers of free electrons will be produced by the inner conductors and outer shield of the prior art cable. Many of these free electrons make their way to many electron traps in the dielectric at both the conductor-insulator interface and at the shield-insulator interface. Most of these latter, previously free, electrons will be stored, that is trapped, in electron traps in the prior art cable.
Trapped electrons occur as a result of excess free electrons transferring from the metal, namely conductors and shield, to the dielectric layer, that is insulator. The presence of gaps and spaces between the metal and the dielectric layer then prevent these excess electrons in the dielectric layer from returning to the metal to recombine with positive ions, namely holes, and hence they are trapped in the dielectric layer.
The spatial distribution of trap-sites and the differential number of trapped negative charges, at or near the shield-dielectric interface versus the negative charges at or near the conductor-dielectric interface, induce an electric field, namely an electromotive force, that acts on other electrons in the inner conductors and the outer shield. A pulse of electrical current called Systems-Generated-Electromagnetic Pulse, or SGEMP, will automatically flow through any conductive path from the inner conductors to the shield, or from the shield to the inner conductor, to balance the surge of displaced charges and to eliminate the electric field. When such a cable interconnects electronic equipment, a transient SGEMP current pulse is said to flow through the circuits that are the conductive pathways inside the electronic equipment between the shield and the conductors. An SGEMP current pulse, of either positive or negative polarity, can damage sensitive electronic components along its path. Such a path is shown in FIG. 2.
When any electrical cable is exposed to X-rays or gamma-rays, all of the materials in the cable become ionized, creating electrons and positive ions, that is, holes. The electrons, being very small and mobile, scatter in all different directions but primarily away from the direction of the radiation source and backscatter toward the radiation source.
The atomic number (Z) of an atomic element indicates the number of electrons in electron shells of an non-ionized atom. Although all materials ionize under radiation, materials with higher atomic numbers have larger radiation cross-sections, that is larger radiation interactivity, which cause them to emit more electrons. Since electrical cables are essentially concentric layers of different atomic number elements, there is a net flow of electrons from the higher atomic number layers to lower atomic number layers, upon irradiation of the layers.
As conductors and shields are commonly made from high-atomic-number metallic elements, namely 47 for silver and 29 for copper, while dielectric insulators are typically combinations of low-atomic-number elements, namely 1 for hydrogen, 6 for carbon, 7 for nitrogen, 8 for oxygen, or 14 for silicon, the electron emission rates are much higher from the conductors and shields than from the dielectrics. Therefore, under ionizing radiation, there is a net flow of freed electrons from the conductors and shield to the insulator. The greater the difference in atomic number of the conductor & shield material from that of the insulator material, the greater the number of electrons available to be trapped at an interface between conductor material and insulator material and at an interface between shield material and insulator material.
In the prior art cable, the difference in electron emissions between the copper wire and its silver coating is inconsequential because they are both metals in direct electrical contact. The electrons flow freely back and forth between the two metals to maintain equal potentials. What impacts the SGEMP current pulse is the difference in emission rate from the metal surface versus the adjacent dielectric. In the prior art cable, that critical interface is between the silver coating on each of the conductor and shield wire, and the dielectric insulation. The very high electron emission rate of silver, relative to that of the dielectric, causes a large net flow of freed charges from the silver toward the dielectric in the prior art cable.
These emitted electrons are sufficiently energetic to jump across any gap that exists between the conductor and insulator, as well as between the shield and the insulator. The greater the gap size, the greater the range an electron travels to its trap-site in the insulator, and the greater the electromotive force exerted by that charge, on the other electrons in the shield and conductor, to drive the SGEMP current pulse.
A transient phenomenon occurs during radiation, known as "radiation-induced-dielectric-conductivity", where there is a certain degree of charge mobilization even within the dielectric. The degree of "conductivity" is material-dependent. Radiation-induced-dielectric-conductivity permits some of the displaced electrons to return to their emission source and recombine with the positive ions, thus curbing the SGEMP driver. However, gaps between the metal and the dielectric insulator eliminate many return paths for charge recombination. The gaps cause many of the electrons to be trapped far away from the metal source, thereby intensifying the electromotive force on other charges within the conductors and shield.
Since this quasi-conductive state in the insulator ends shortly after the radiation pulse, free electrons can travel only a short distance within the insulator before becoming trapped. Thus, the electron trap-sites are generally located close to the surface, that is within an electron range, of the insulator at each metal-insulator interface.
By convention, current in a normal circuit is considered to flow from a more positive point to a more negative point, even though the actual electron flow is in the opposite direction. Positive current flows from a signal source, namely conductors, to the ground, namely shield, while the electrons flow from the shield toward the conductors.
The prior art cable has gaps at both metal-insulator interfaces. The gap sizes at the shield-insulator interface are much larger than those at the conductor-insulator interface. Furthermore due to the shield braid wires having a greater electron emission surface area, the shield emits more electrons than the conductor wires, even though both shield and conductors are made of the same material. Thus, more electrons are trapped at the shield-interface, and trapped further from the shield, than their counterparts at the conductor-interface. Since the replacement electrons are drawn, from the conductors through intervening circuits, toward the shield, a negative SGEMP current pulse flows from the shield to the conductors in the prior art cable.
Each material layer in a cable, in the path of the radiation, attenuates the intensity and alters the energy spectrum of the radiation traveling through it. The degree to which one material shields those materials behind it, depends upon its material composition, that is its atomic number, density, thickness, coverage, and the wavelength of the radiation involved. Multiple conductors twisting around each other within a shielded cable would provide some degree of self-shielding which could limit electron emissions per conductor. Therefore, in the prior art cable, the SGEMP current pulse per conductor for a 3-conductor cable is less than that for a 2-conductor cable. Likewise for a 4-conductor cable the SGEMP current per conductor is less than that for a 3-conductor cable. However, the SGEMP current per conductor for a 2-conductor cable is not less than that for a single-conductor cable. This is because a shield that spans two twisted-insulated conductors inherently leaves larger gaps between the shield and the insulated conductors than a shield over a single-insulated conductor. On balance, gap sizes have a far greater impact on SGEMP current than X-ray or gamma-ray attenuation through self-shielding.
To summarize, X-rays and gamma-rays cause electrons to be displaced from the shield braid and from the conductors. Depending on the particular cable design, material geometry, gaps, radiation attenuation through materials, and the type of materials involved, more electrons are generally trapped at one interface than the other. This imperfect matching, of the forward-emitted shield wire electrons versus the reverse-emitted conductor core wire electrons, causes a charge imbalance. A resulting replacement current flows from the shield to the core wire, or from the core wire to the shield, through interconnected electronic packages. The polarity of this SGEMP current indicates the direction of current flow. Such transient negative or positive current, passing through electrical circuits, can damage sensitive electronic components inside the electronic box or equipment.
The smaller the charge imbalance in a given cable design, the smaller the SGEMP current pulse, and the lower the potential for damaging the interconnected electronics. A cable designed to have a very low SGEMP current pulse response to X-rays or gamma rays is considered to be "radiation-hardened". The prior art electrical cable has both high electron emissions and large gaps, that cause many electrons to be trapped at a distance. Together they generate the large electromotive force that induces a substantial SGEMP current to flow. Consequently, the prior art cable is not radiation-hardened.
A disclosed cable is a radiation-hardened electrical cable. The electrical cable achieves radiation-hardness by the insertion of low-Z trapped-electron reducers. The trapped-electron reducers reduce the emission of electrons between a high-Z metallic conductors and insulator, and the emission of electrons between a high-Z metallic shield and insulator when the cable is irradiated by high energy photons. The trapped-electron reducers minimize gaps which reduce the electron range to trap-sites and enhance charge recombination.
A disclosed cable can have a high-Z inner conductor, a first low-Z trapped-electron reducer around each inner high-Z conductor, a dielectric insulator layer around that first trapped-electron reducer, a second low-Z trapped-electron reducer around a twisted-bundle of insulated-conductors (or around each insulated conductor), and a high-Z outer shield around the second trapped-electron reducer(s). A dielectric protective jacket can be placed around the outer shield.
The two trapped-electron reducers in the disclosed cable are a matched set, that is the reducers have identical dielectric and metals, to equally reduce electron emissions at both conductor-insulator interface and shield-insulator interface. Each trapped-electron reducer is made from a low-Z metal layer, such as an aluminum layer, that is joined to a low-Z dielectric film, such as a mylar or Kapton film. The aluminum layer can be an aluminum foil or aluminum film. The dielectric film is laminated to the aluminum layer as a dielectric backing, thus forming an essentially gapless-interface. The dielectric film can be made of identical dielectric material to the low-Z dielectric insulator.
More specifically, the disclosed cable can have one or more inner silver-coated copper central wire conductor(s). Each conductor may be a single silver-coated copper solid wire or a bundle of twisted-silver-coated-copper wire strands. A first trapped-electron reducer is around each inner silver-coated copper wire conductor, with the aluminum layer of the first trapped-electron reducer in direct electrical contact with the inner silver-coated copper wire conductor. A dielectric insulator is around each of the first trapped-electron reducers, with the insulator in contact with the dielectric backing of the trapped-electron reducer. A second trapped-electron reducer is around a twisted-bundle of insulated-conductors (or around each insulated-conductor), with the dielectric backing of the second trapped-electron reducer in contact with the dielectric insulator. A silver-coated copper braided shield can be around the second trapped-electron reducer, with the metallic shield in direct electrical contact with the aluminum layer of the second trapped-electron reducer. A protective dielectric jacket, can be placed around the metallic shield.
As mentioned before, the difference in electron emissions between a copper wire conductors and its silver coating is inconsequential because they are both metals in direct electrical contact. The electrons flow freely back and forth between the two metals to maintain equal potentials. Likewise, the difference in electron emissions between the silver-coated wire and the aluminum film in the trapped-electron reducer is inconsequential because they are both metals, also in direct electrical contact and at equal potential.
Again, what impacts the SGEMP current pulse is the difference in electron emission rates of the metal versus the dielectric at their mutual interface. For the disclosed cable, that critical interface is, within each of the trapped-electron reducers, between the aluminum layer and its dielectric backing. Since the dielectric backing is laminated to the aluminum layer, the volume (number and size) of gaps at their mutual interface are orders of magnitude less than the gaps in the prior art cable.
Electrons emitted by the inner conductors and the outer shield will be electrically conducted back to the inner conductors and the outer shield from the aluminum layers of the trapped-electron reducers and not travel to electron traps in the dielectric layer.
Electron emissions from the dielectric film of each of the two trapped-electron reducers of the disclosed electrical cable will tend to equalize. This is the case since the material that makes up each of these two dielectric films is identical to each other. Further, said materials can be identical to material that makes up the dielectric insulator layer. Therefore there would be no net electron flow between each trapped-electron reducer's dielectric film and the dielectric insulator layer, so any gaps between them would have no impact on the SGEMP response in the disclosed cable.
Further, the inner conductors and outer shield of the disclosed cable will not send as great a number of free electrons toward the dielectric backing and insulator layer, due to protection against X-rays or gamma rays afforded by the aluminum layer of each of the trapped-electron reducers of the disclosed cable. Without such protection, the inner conductors and outer shield would send many more free electrons toward the dielectric backing and insulator layer.
The aluminum layer of each of the trapped-electron reducers, the protected inner conductors and the protected outer shield, all taken together, will not send as many free electrons toward traps in the dielectric insulator, as compared to unprotected inner conductors and outer shield of a prior art cable.
A table below shows the relative electron emission rates for disclosed cable materials when the materials are irradiated by high energy photons having energies ranging from 20,000 to 40,000 electron-volts. The electron emission rates produced in response to these high energy photons have been normalized to the aluminum emission rate. In other words, the emission rate of each element is indicated as a multiple or faction of aluminum emission rate. Such high energy photons, typical in nuclear explosions, produce a smaller SGEMP current pulse in the disclosed cable than in a prior art cable.
The electron emission rate of aluminum, aluminum having an atomic number, namely Z, equal to 13, is an order of magnitude less than that of silver, wherein Z equals 47. The average Z of the dielectric material, such as mylar or Kapton, used for the dielectric insulator layer and dielectric film of the disclosed cable, is less than 6. The emission rate of aluminum is much closer to the emission rate of the dielectric material in the disclosed cable. The smaller the difference in electron emission rates at the critical interface between the metal and its adjacent dielectric, the fewer electrons would be displaced under radiation. There is an order of magnitude reduction in electron displacements at the critical interface in the disclosed cable, that is aluminum-to-dielectric interface, versus the prior art cable that has a silver-to-dielectric interface. Less electrons displaced means less electrons available to be trapped.
TABLE
______________________________________ Relative Electron Atomic Emission from 20 to Element Number (Z) 40 keV Photons ______________________________________ (I) Typical metal material of conductors and shield Silver (Ag) 47 11 Copper (Cu) 29 9 (II) Metal material of the layer of trapped-electron reducer Aluminum (Al) 1 3 1 (III) Typical dielectric materials of insulator layer and film of trapped-electron reducer Mylar (H,C,O) 1,6,8 (average 0.1 less than 6) Kapton (H,C,N,O) 1,6,7,8 (average 0.08 less than 6) ______________________________________
Given a prior art cable where the shield and the conductors are made of the same material, the braided shield, having a geometrically larger electron emission surface area, would emit more electrons than would the conductors. In the prior art cable, this geometric imbalance contributes to the large negative-polarity SGEMP current pulse response.
In the disclosed cable, the trapped-electron reducers are low-electron emitters, essentially gap-free, and the emission surface areas of the shield-aluminum interface versus the conductor-aluminum interface are more balanced geometrically. This leads to a reduction in electron trap-sites and lower SGEMP response in the disclosed cable.
Since a smaller number of electrons will be stored in the dielectric layer of the disclosed cable, than in a prior art cable, there will be a smaller electromotive force (EMF) arising from the dielectric layer. A smaller SGEMP replacement current will be created in the inner conductors and outer shield. There will be a smaller SGEMP current pulse produced in the disclosed cable, as a result of inserting a gap free, low-Z metal-dielectric interface, such as an aluminum-dielectric layer, between the high-Z inner conductors and its primary insulation, and also inserting a second gap free, dielectric-low-Z metal interface between the primary insulation and the outer high-Z metal shield. Electrical equipment connected to a cable with high-Z conductors and shield protected with trapped-electron reducers will not be harmed. Again a smaller Systems-Generated-Electromagnetic Pulse will be produced in the disclosed cable, than in the prior art cable.
A disclosed cable having multiple inner conductors can have two alternate constructions. Both constructions begin with a first trapped-electron reducer around each inner conductor. (A conductor can be a single silver-coated copper solid wire or a bundle of twisted silver-coated copper wire strands.) The aluminum layer of each inner trapped-electron reducer is in electrical contact with each inner conductors. A dielectric insulator layer is around each of the inner trapped-electron reducers.
Construction 1 has one outer trapped-electron reducer around the twisted-pair of dielectric-insulated-conductors. The dielectric film of that outer trapped-electron reducer is in contact with the dielectric insulator layer of both insulated-conductors. A metallic shield is around this outer trapped-electron reducer. A protective nonconductive jacket is around the metallic shield.
Construction 2 has an outer trapped-electron reducer around each individual dielectric-insulated-conductor. The dielectric film of each outer trapped-electron reducer is in contact with the dielectric insulator layer of each insulated-conductor. A metallic shield is around both outer trapped-electron reducers. A protective nonconductive jacket is around the metallic shield.
Construction 1 will have a smaller diameter than construction 2 because it uses one outer trapped-electron reducer for all the insulated-conductors rather than one outer trapped-electron reducer for each insulated-conductor. In the application of twisted two-conductor cables, representing signal and return lines, construction 1 is more effective in minimizing crosstalk in cable bundles.